This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
The photolithography technique/process is a manufacturing technique that enables to transfer a geometric pattern from a photomask to a photoresist substrate (usually a light sensitive polymer substrate), when the photomask is illuminated by a light illumination unit (i.e. when the photomask is exposed to light illumination unit). Such technique/process is widely used in the semiconductor industry especially for the fabrication of integrated circuits.
In order to design more complex and smaller integrated circuits (as they are dedicated to be used in more and more compact electronic device such as tablet, mobile phone, and so on), a natural trend in the semiconductor industry is to improve the photolithography technique/process in order to be able to transfer a geometric pattern at very small scale (i.e. even at Nano metric scale).
Resolution of the photolithography process, i.e. the smallest feature of the geometric pattern that can be created on a photoresist substrate, is determined by the resolving capability of the photoresist and by the optical resolution of the imaging system. The latter is determined by the wavelength of the imaging light (λ) and the numerical aperture (NA) of the projection lens. For a refractive or reflective type optical projection unit, it typically does not exceed one wavelength of the incident light in the host medium. The resolution can be improved by decreasing the wavelength or increasing the numerical aperture. The latter is possible for contact printing (with a mask put in direct contact with the photoresist substrate), however, it leads to mask degradation and thus should preferably be avoided. Aiming at increasing the resolution, several techniques have been proposed.
For example, in the book entitled “Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies”, the chapter 8, entitled “Nanolithography”, written by Gunasekaran Venugopal and Sang-Jae Kim, describes and compares several techniques for manufacturing integrated circuit comprising nano-geometric patterns. The document US 2016/0147138 depicts a particular photomask used in a particular lithography technique (the EUVL (which stands for Extreme Ultraviolet Litography)). Another technique for manufacturing integrated circuit comprising nano-geometric patterns is described in the article entitled “Nanoscale materials patterning and engineering by atomic force microscopy nanolithography” by X. N. Xie, H. J. Chung, C. H. Sow, and A. T. S. Wee.
In order to design more complex nano-geometric patterns on a photoresist substrate, a technique described in the document US2016/0259253 is proposed based on near-field focusing due to the surface plasmon phenomenon. This technique enables a super resolution order of a fraction of the incident wavelength. However, this technique relies on the use of a multi-layer metal-dielectric mask, whose fabrication cost and complexity may not be acceptable for some applications.
Another technique named the nanosphere photolithography (NSP) technique (introduced in the article entitled “A deep sub-wavelength process for the formation of highly uniform arrays of nanoholes and nanopillars” by W. Wu et al., published in Nanotechnology 18, 2007) provides a cost-effective solution for fabrication of nanostructures with feature dimensions below 100 nm. This technique exploits nanojet beams produced by dielectric spheres (as detailed in the article entitled “Photonic nanojets” by A. Heifetz et al. The use of a monolayer of nano- (or micro-) spheres deposited on top of an optically sensitive material (i.e. photoresist) enables one to achieve a highly selective exposure of the photoresist layer. When exposure under UV light, the hot spot size produced by nanojet microspheres is about a half wavelength in the photoresist material. In such a way, microstructures with features size order of 100 nm can be fabricated. Hence, nanosphere lithography (NSL) is an economical technique for generating single-layer hexagonally close packed or similar patterns of nanoscale features (see for example the article entitled “Fabrication of nanopillars by nanosphere lithography” by Cheung et al. in Nanotechnology 2006). Another example of application of nanosphere lithography is depicted in FIG. 2 of the article entitled “Photon nanojet lens: design, fabrication and characterization” by Chen Xu et al., published in Nanotechnology 27 (2016).
Even if the nanosphere photolithography (NSP) technique is effective in terms of cost and throughput capacity, it may suffer from the following drawbacks:                it has a poor reproducibility caused by certain difficulties with a precise positioning of microspheres,        Fabrication tolerance caused by the imperfection of the microsphere shape, size, and positioning,        Limited diversity of microstructure shapes that can be fabricated (for the moment, it is limited to simple geometries, like circular nanoholes, nanorings, and nanopillars).        
Hence, there is a need to provide an alternative technique to the nanosphere photolithography technique that may overcome some of these limitations.